[JASJAR, XDA Exec, MDA Pro ROM Development] -Jonny-:
types of memory

Ok, quick resume of memory:

There are four types of internal memory on the Universal (the quoted MB are from a 64MB G3 uni):

[B]Storage Memory[/B]
Type: Internal Storage
Size: Usually 43.72MB with 40MB free (depends on installation of full/vanilla ROM)
What: Your main storage for all installations on your device (including windows)
Use: This where the device ROM (eg WM6.x) and other device programs are stored (everything under \).
eh? : This can be considered much like your C: drive on a PC.
+info: Get a ROM that has just about everything you want.
Getting vanilla ROM and installing apps in Storage is likely to use more Storage than if you get ROM that has all apps you require.
I never install anything here unless I have no option, or it's a Today item.
Some programs will use this as cache or temp storage area.

[B]Program Memory[/B]
Type: Program (RAM)
Size: Usually 48-49MB with 30MB or less free (this figure lowers with the more Apps that are run, an initial 12MB is eaten by Windows)
What: This is your available memory for running programs. Program Memory is also for used by Boot and PagePool (usually 6MB).
Use: The more of this you have, the more programs you can run, and the faster memory hungry apps can run.
eh? : This can be considered much like your RAM on a PC
+info: The Today Screen items will also eat up this memory, so stay away from "bling" Today items if you want other memory hungry apps to run (like route navigators)
NB. This memory is NOT eaten up or reduced by any other medium (ie, FlashDisk). 30MB free is great, but 20MB is usual running for a 64MB Uni.

[B]FlashDisk[/B] (only available on G3 according to wiki [I]- someone clarify?[/I])
Type: Internal Storage
Size: Can be 10MB to 30MB - and is made available solely by what's left by the ROM and given to you by the cook
What: Extra storage which is retained through a soft-reset.
Use: Store programs you always want available (ie, not on SD), but don't want in your Storage Memory
eh?: This can be considered much like your second hard-drive on a PC.
+info: If important, choose cook and ROM wisely, otherwise install such stuff on approprate SD/Storage.
The FlashDisk is where I store my messages and attachments (and programs like music/video players if cook doesn't supply them).
NB. This doesn't take up any Program Memory. It's either made available by the cook, or if the cook doesn't make it available it's wasted space that you can't use.

[B]ExtendedROM[/B]
Type: Internal Storage
Size: Is usually 9.96MB
What: Extra memory which doesn't survive a soft-reset.
Use: Anything you don't mind losing :)
eh?: This could be considered as a RAMDrive on a PC (that is, in that it's volatile, but not that it takes up any Program Memory)
+info: I use this for all cache (eg PIExplorer, Opera etc)
NB. Again, this is either made available or wasted space you can't use (see FlashDisk NB).

[B]Storage Card[/B]
Type: External Storage
Size: Can be anything (I have seen up to 16GB)
What: Extra storage for programs, cache etc. Always slower than any of the above. (NB. If anyone really interested, I can get you some values here)
Use: Anything that doesn't fit into the Uni ;)
Install apps here that aren't required by the system (ie, non-Today items)
eh?: This could be considered as any removeable storage on a PC (even an SD card!!!)
+info: Don't put anything here that the system expects to exist (eg, Today items). I use it for maps, music, videos and all apps not supplied by ROM cook.
I only have use one huge SD card, so I install all my apps on it. You may want to reconsider your installations if you use more than one SD card.

However, Cotulla created a ROM for G3 that combined Storage (not program memory)

.